1. Field of the Invention
The present invention relates to a technique for adjusting a transmission rate of, for example, Moving Picture Experts Group-2 (MPEG-2) data. More particularly, the present invention relates to a method and an apparatus for adjusting a transmission rate of MPEG-2 data, in which the data is being output at a constant output transmission rate that is maintained despite receiving input MPEG-2 data at various input transmission rates, the recovery of data without any information regarding clock data recovery (CDR).
2. Description of the Related Art
An MPEG-2 system is adapted for two particular types of multiplexed bit streams, namely a program stream and a transport stream. With regard to these two streams, a program stream is utilized for a multiplexing method either which is applied when one broadcast program including video, audio, and captions is used in an error-free channel environment or when an error correction function of a medium's own is used in tact, as in a compact disks CD or DVD's. A transport stream is utilized for a multiplexing method used when several broadcast programs are simultaneously transmitted in a channel environment having that is not error free. In other words, the program stream is used for storing one program, such as a video CD, and the transport stream is used for digital broadcasting of a plurality of programs by means of an artificial satellite.
The MPEG-2 system employs a packet multiplexing method, which has been previously used in a time division multiplexing (TDM) method. Herein, each of video and audio bit streams is first divided into packetized elementary streams (hereinafter, referred to as ‘PESs’) called packets having an appropriate length (in terms of kbytes). Each of the PES packets has an agreed to length limitation of no more than 64 kilobytes maximum, so as to be to communicate with various applications, and may take any one of a fixed length and a variable length. Also, each PES packet may be transmitted either at a variable transmission rate or intermittently. The program stream and the transport stream are made by multiplexing each PES into one bit stream.
A length of a packet depends largely upon the transmission channel or the medium used. For example, in an asynchronous transfer mode (ATM), which is a protocol for a broadband integrated services digital network (BISDN), packet cells of 53 bytes are used. In such a packet cell, since a header for containing basic information occupies 5 bytes, actual information (payload) for a user is contained within 48 bytes. The transport stream packets (hereinafter, simply referred to as “transport packets”) have a comparatively short fixed length of 188 bytes in consideration of connectivity to the ATM. In the transport packet, the first 4 bytes make up a header, and 184 bytes are a portion for user information carrying video or audio data. Such a transport packet is carried in 4 ATM cells and is transmitted. At this time, one byte of the 48 bytes for user information in each ATM cell is used for an ATM adaptation layer (AAL), and thus actual user information is contained in 47 bytes.
The transport packets, which have been first multiplexed and transmitted from each broadcasting station, are re-multiplexed to construct one bit stream, thereby being arranged in a form that can be transmitted through one repeater. Multiplexing of digital satellite broadcasting includes time division multiplexing (TDM) and frequency division multiplexing (FDM). In other words, repeaters have a bandwidth of 27 MHz and are managed in a form of the FDM, but 4 broadcasting stations share the repeaters in the TDM method. The transport stream to be carried to each repeater then undergoes a Reed-Solomon coding and a convolutional coding, and is transmitted between a ground station and a satellite by a quadrature phase shift keying (QPSK) modulation.
A decoding of the transport stream by a receiving terminal is performed with a reverse process to above-mentioned process. First, the receiving terminal selects a repeater including a broadcast to be desired by a user of the receiving terminal, then the terminal performs a QPSK demodulation of a signal input through the repeater, and corrects the error. The repeater outputs a bit stream in which several broadcasts are multiplexed. Therefore, the receiving terminal first chooses transport packets of only a broadcasting station which is desired to be received, decodes video packets from among the chosen transport packets with a video decoder, and audio packets from among the chosen transport packets with an audio decoder, respectively, thereby reproducing video and audio.
Recently, the transmission of video data through broadband communication systems, such as a cable TV, a satellite TV network, etc., has become extremely popular. With such a video data transmission, a source video sequence is encoded either after compression (or without compression) to form transport streams, and then may be transmitted at a proper constant bit rate (CBR) or at a variable bit rate (VBR). However, for various applications, before communication is performed to a set-top box (STB) and/or intermediate nodes of a network, an apparatus for varying a transmission rate of video data is necessary. Therefore, an apparatus capable of processing all of the video data of various transmission rates can be contained in the set-top box or the like, but this construction is far from being desirable in terms of the cost and such an apparatus has a complex construction.
In addition, in order to vary a transmission rate of MPEG-2 data, the use of a REMUX or the like having a comparatively high price is required. Moreover, even with a digital video broadcast asynchronous serial interface (DAB-ASI) as a widely-used transmission method for the transmission of MPEG-2, it is impossible to know the actual transmission rate of data, as only an interface clock of the DVB-ASI can be extracted. Therefore, clock information of the DVB-ASI is also used as clock information of the data, so that an underflow or an overflow may occur in a buffer of a receiving section, wherein the received data are stored in a buffer, such as an FIFO (First In First Out), and subsequently transmitted. For example, when the difference between an actual data rate and the clock information is 1 ppm on the basis of 27 Mbps, either an underflow or an overflow occurs once per approximately 330 seconds. Also, when the difference is 2 ppm on the basis of 27 Mbps, either an underflow or an overflow occurs once per approximately 165 seconds (330/2=165). Therefore, there is a need in the art to provide efficient transmission the MPEG-2 data and prevent the problematic data losses caused by clock mismatches between a transmission time and a receipt time when the MPEG-2 data are transmitted.